1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image by jetting droplets of a liquid such as an ink from a plurality of nozzles toward a recording medium, while moving a head unit having the plurality of nozzles, and relates to the head unit.
2. Description of the Related Art
As it is shown in FIG. 6A and FIG. 6B, an ink-jet printer, which forms an image on a recording paper 82 by jetting ink droplets from a plurality of nozzles toward the recording paper 82 while moving a jetting head 81 in a scanning direction, has been used practically. In this ink-jet printer, when the jetting head 81 is moved, air enters relatively between the jetting head 81 and the recording paper 82 from one end of a movement direction of the recording head 81, and this air flows in one direction of the scanning direction. Since both sides orthogonal to the scanning direction between the jetting head 81 and the recording paper 82 (in other words, both sides in a transporting direction of the recording paper) are open, thereafter, the air escapes to the both sides, in the transporting direction, at which a resistance is low (FIG. 6A is a plan view explaining this state). As a result, an air current A, which spreads over the both side in the transporting direction while flowing in one direction of the scanning direction, is formed between the jetting head 81 and the recording paper 82, on both sides in the scanning direction and nearby.
Moreover, generally, it has been known that ink droplets which are jetted from the nozzle include a main droplet 83b which is accompanied by a satellite droplet 83a having a weight smaller than the main droplet. Since the main droplet 83b is heavier in weight, the main droplet 83b is not so affected by the air current A, and flies almost straight from the nozzle. On the other hand, the satellite droplet 83a jetted from the nozzles on both sides in the transporting direction flies to be deflected in a direction in which the air current A spreads. Consequently, as shown in FIG. 6B, the satellite droplet 83a jetted from the nozzle at an intermediate portion in the transporting direction while flying to be deflected lands on the recording paper 82 at a position which is horizontal in the scanning direction of the main droplet 83b. On the other hand, the satellite droplet 83a, in the air current A which spreads toward both sides in the transporting direction, lands at a position which is shifted to be inclined with respect to the main droplet 83b. As a result, relative positions of landing of the satellite droplet 83a and the main droplet 83b are different between near the center position of the transporting direction and both sides of the transporting direction. Therefore, a dot which is formed by the satellite droplet 83a and the main droplet 83b is shifted from a desired position of landing, and a diameter and a shape of the dot are not uniform, thereby causing a defective image quality.
When the main droplet 83b is fine, or a jetting speed is slow, there is a possibility that the main droplet 83b is affected by the air current A. In this case, there is a possibility that the defective image quality becomes more substantial.
Moreover, when ink droplets are jetted from a nozzle, not only the main droplet 83b and the satellite droplet 83a, but also more fine ink particles called as a mist are known to be formed. It is considered that the mist is generated by an air current between the jetting head and the recording paper, when the ink is ejected from the nozzle and divided into the main droplets 83b and the satellite droplets 83a. Moreover, the mist is floated by the air current which is generated by the movement of the jetting head.
For preventing the mist and the satellite droplets of the ink from being floated over a wide range, in a jetting head according to a conventional technology, a discharge port which is capable of discharging air toward the recording paper is formed at a front side in the scanning direction. For example, in a recording head described in US Patent Application Laid-open No. US-2002089563 (corresponds to Japanese Patent Application Laid-open No. 2002-273859), an air curtain is formed at a front side in the scanning direction with respect to nozzles by discharging air from the discharge port. Due to the air curtain, an air current between the jetting head and the recording paper, which is formed when the jetting head is moved, is blocked, and the mist and the satellite droplets of the ink are prevented from being floated over a wide range.
However, in the recording head of the conventional technology, the space between the jetting head and the recording paper is open in the scanning direction and the transporting direction. According to such a structure, an air curtain is formed and air is intercepted at the front side of the jetting head. However, air is blown between the jetting head and the recording paper from both sides of the transporting direction, and a vortex flow is generated at a rear side of the air curtain. The vortex flow disturbs the air current between the jetting head and the recording paper, and the liquid droplets land at an undesired position of the recording paper. In this manner, although the jetting head of the conventional technology is capable of preventing the floating of the mist and the satellite droplets of the ink over a wide range, it is difficult to land a dot accurately at a desired position, and to make uniform a diameter and a shape of the dots.